1. Field of the Invention
The present invention relates to a method and a device for the localization, from an aircraft, of fixed or mobile terrestrial transmitters. These transmitters belong to a system of TDMA type cellular radiocommunications. In particular, this system consists of a radiocommunications network using the GSM or DCS 1800 system. The DCS 1800 standard is a development of the GSM standard. The acronym TDMA is an abbreviation of the term  less than  less than Time Division Multiple Access greater than  greater than . It is sometimes replaced by the acronym AMRT which is an abbreviation of the French term  less than  less than Accxc3xa8s Multiple Rxc3xa9partition dans le Temps greater than  greater than . The acronym GSM is an abbreviation of the term  less than  less than Global System for Mobile communication greater than  greater than . The acronym DCS is an abbreviation of the term  less than  less than Digital Cellular System greater than  greater than .
The GSM network makes it possible to set up digital communications, between mobile units or between mobile units and subscribers to the public switched network.
To enable the network to offer these services, a series of functions has been defined. These functions are those required of any network of mobile units. Such functions include, for example, dialling, routing towards a mobile user, cell transfers, etc. These functions are distributed among entities.
2. Description of the Prior Art
The full network, shown schematically in FIG. 1, includes several of these entities. An entity may be:
A Mobile Station or Mobile Stations.
The mobile station is the physical equipment used by the user of the GSM network to access the telecommunications services on offer. There are various types of mobile stations: these include mobile stations mounted on vehicles and portable units. Portable units currently account for the major part of the market for mobile stations. Mobile stations (MS) can move through a certain number of cells.
A Radio Sub-System or BSS (Base Station System).
The Base Station System is the equipment that covers a given geographical area known as a cell. The BSS contains the hardware and software needed to communicate with the mobile stations. Functionally, this Base Station System is divided between a control function carried out by a Base Controller Station or BSC and a radio transmission function carried out by the Base Transceiver Stations or BTS.
Each cell is covered by means of a base transceiver station that manages the connection with the mobile station using an interface called a xe2x80x9cradio interfacexe2x80x9d.
The relations between the base transceiver stations BTS and the base station controllers BSC are defined by xe2x80x9cthe Abis interfacexe2x80x9d. The base station controllers BSC are connected to the rest of the GSM network by means of the xe2x80x9cA interfacexe2x80x9d.
A xe2x80x9cNetwork and Switching Sub-Systemxe2x80x9d (NSS).
The NSS consists of three elements:
the MSC (xe2x80x9cMobile services Switching Centerxe2x80x9d): this is the switching center of the mobile service in charge of the routing of the communications to and from the mobiles in a cell,
the HLR (xe2x80x9cHome Location Registerxe2x80x9d): this is the data base in which the permanent parameters of a subscriber are recorded; the HLR furthermore contains constantly updated information on the localization of the mobile unit,
the VLR (xe2x80x9cVisitor Location Registerxe2x80x9d): this is the data base in which there is recorded the precise localization of the mobile unit in the calling zone.
FIG. 2 illustrates a typical configuration of use of mobile stations MS, base transceiver stations BTS and a base station controller BSC. Several mobile stations MS are managed by one base transceiver station BTS. Several base transceiver stations BTS are connected to a single base station controller BSC.
The structure of the network is of the cell type, i.e. the capacity of the network is obtained by making a grid of the territory by means of cells that are each served by a base transceiver station. A major characteristic of a cell network is the re-utilization of the frequencies through the network A concept associated with this characteristic is that of the pattern of re-utilization. The pattern of re-utilization defines the number of juxtaposed cells that do not use the same frequencies.
FIG. 3 shows an exemplary mode of planning for the use of frequencies with a Size 4 pattern of re-utilization. In this example, two cells using the same frequency, for example the frequency having the number 3, are at a distance of at least one cell from each other.
The size of the cells may vary from a hundred meters to several tens of kilometers. An aircraft that flies over the cellular radiocommunications network is likely to intercept the signals sent out in several cells and even the signals sent out by cells using the same frequency. Thus, the problem arises of knowing how to localize the different transmitters from the aircraft.
To this end, an aim of the invention is a method for the real-time localization of terrestrial transmitters belonging to a TDMA type cellular radiocommunications network. The method consists of:
the interception, from an aircraft, of radioelectric signals and the detection of these signals by the implementation of a multichannel synchronization to determine the various sources of transmission,
the simultaneous computation of an elevation angle xcex94 and an azimuth angle xcex8 to instantaneously determine the direction of arrival of the radioelectric signals from the determined transmission sources,
the building of tracks in the geographical areas in which the transmission sources are located in order to estimate the position of the transmitters.
The method consists of the use of an antenna array distributed over an aircraft and of the association of techniques of multi-sensor synchronization with a technique of high-resolution direction finding in order to carry out the real-time geographical localization of terrestrial transmitters of a TDMA type cellular radiocommunications system.
From the signals received on a network of distributed antennas, the invention synchronizes the received transmissions. The received transmissions can arrive simultaneously, whereas they will have been sent out by different mobile stations. The method of synchronization is associated with a recognition function to enable the detection of the transmissions coming from one and the same source in spite of possible overlapping between the various transmissions coming from different sources. After the recognition of the transmission sources, the method according to the invention carries out an operation of high-resolution direction finding. The high-resolution direction finding provides information on the elevation and azimuth angles of the direction of arrival of the signals sent by the different sources, in the reference frame of the aircraft.
The aircraft moves with reference to the absolute geocentric reference to which terrestrial transmitters are related. It moves at a certain attitude. The attitude is characterized by instantaneous angles of roll, pitch and yaw. The method uses information from the inertial guidance unit of the aircraft in order to convert the direction of arrival of the signals sent by the transmission sources from a reference system of the aircraft into the absolute geocentric reference, and to compute the intersection of their direction with the earth by using a digital ground model; the points of intersection are called elementary plots. In the geographical areas in which sources are present, the method builds tracks. The positions of the transmitters are extracted from the tracks, for example by locating the maximum density of the track, or for example by computing a weighted barycenter of the elementary plots of each track.
Since the GSM transmitters can function in frequency-hopping mode, it is necessary to be able to carry out localization operations with a small number of measurements. The use of techniques based on a density of probability enables the extraction of reliable localizations. This is done by taking account of errors on only some elementary plots.